1. Field of the Invention
The present invention relates to improvements in compact, low frequency underwater electro-acoustical transducers which employ the advantage of the low resonant frequencies of the flexural modes of the discs, and more particularly to apparatus of a type in which two piezoelectrically excited discs are mounted at opposite ends of a fluid-tight chamber with a fluid medium contained in the chamber having an acoustical mismatch relative to the discs so that radiation from the inner faces of the discs is minimized.
2. Description of the Prior Art
Transducer apparatus of the type referred to has been described, and the theory thereof discussed in a U.S. Navy Underwater Sound Laboratory Research Report No. 490 "Theory of the Piezoelectric Flexural Disk Transducer with Application to Underwater Sound", by Ralph S. Woollett (Armed Services Technical Information Agency Catalog No. 249,731). The apparatus employing two flexure discs is particularly described therein at pages 58-60, and page 63 (where a short treatment of an embodiment using a continuous trilaminar disc is briefly described). The use of this type of apparatus has been proposed where a source for radiation of low frequency acoustical energy, i.e. of the range 50 Hz, to 2,000 Hz, is required in mobile vehicles such as acoustic counter measure and target vehicles. For these applications, a transducer apparatus having maximum dimensions of ten or twenty inches and a weight of less of 100 pounds is possible when this type of a construction is employed. Electroacoustic transducers employing acoustic resonant properties (as contrasted with flexure disc resonant properties) for this frequency range are massive, a typical one being dozens of feet in each dimension and weighing several tons.
Prior art double flexure disc transducer apparatuses of the type referred have the limitation of narrow bandwidth, due to the single resonant frequency of the flexure discs. One approach to overcoming the narrow bandwidth has been to use a plurality of transducer apparatuses, having the resonant frequencies of their respective discs spaced over the desired band. However, this produces an additional problem of greater weight. In addition, where the device is to be operated at any significant depth, a pressure equalizing system for maintaining the pressure within the chamber equal to ambient sea pressure is necessary, and the use of several transducers requires a significantly larger compensation system.
U.S. Pat. No. 3,510,698 to F. Massa is of interest. It discloses a double wall flexure type apparatus. Two circular vibratile walls are formed by machining away all but a thin wall in a solid cylindrical workpiece to form a cup-shaped element. Two such cup-shaped elements are put together rim-to-rim, forming an organization of two circular vibratile walls at opposite ends of a cylindrical chamber. Each circular vibratile wall is driven by two concentric piezoelectric laminar units consisting of a central circular unit and an annular band-shaped unit. The circular piezoelectric laminar unit and the annular unit of each wall are driven in opposite senses of radial strain. For example, when the circular unit drives the central region of the circular vibratile wall in compressive strain, the annular unit drives its outer annular region in tensile strain.